1. Field of the Invention
The present invention relates to a printer controller for dot matrix type serial printers having a text processing function and used for word processors, or the like. The printer controller is capable of correcting vertical misregistration of a print in the forward and backward printing.
2. Description of the Prior Art
Heretofore, serial printers having a movable unit (carriage) on which a recording head is mounted are proposed in a number of forms that mount various types of recording heads. The recording heads are adapted to make reciprocating movement perpendicularly to the feed direction of a recording medium such as paper, transparent films for over head projectors, or the like. For example, recording heads such as wire dot heads, thermal heads, thermal ink-transfer heads, ink jet recording heads are used in this type of serial printers, particularly for producing characters, figures and other pictures in a dot matrix.
To make a reduction in cost of word processors having such serial printers incorporated thereinto, all the operations, such as printing, text processing and the control of the floppy disk drives (FDD), are usually controlled by a single microprocessing unit (MPU). The reason is that this configuration reduces the total cost of the system although an excessive load is applied on the software. To attain high speed printing, since a software dependent system has a limit, a system has been proposed in which the print control unit partly uses gate arrays to perform operations at a relatively low cost by means of hardware. For example, the inventors proposed in Japanese Patent Application Nos. 1-344916 and 1-344917 (U.S. Ser. No. 635,870 filed on Dec. 28, 1990 in Art Unit 337), a technique that the print waiting time is minimized when the forward and backward printing is performed by using a line buffer for one line, in which data conversion of the next line is conducted during printing of the line.
Moreover, such as described in Japanese Patent Application Publication No. 57-4952, with a purpose to further reduce the useless motion of the carriage, the carriage is controlled to move only in an area in which print data is provided, and thereby the printer is enhanced in throughput.
In this kind of printer which performs forward and backward printing, there are required capabilities, such as forming tables using vertical and horizontal ruled lines, forming figures such as a graph, printing enlarged characters over lines, and hence it is desired to register vertical relative positions of prints in the forward and backward printing at a higher accuracy. To meet the requirement to improve resolution of print, dots which constitute print patterns becomes rather finer, and thus slight vertical misregistration of print positions in the forward and backward printing becomes conspicuous and provides poor appearance. Also from this point it is strongly desired to exactly adjust prints in the forward and backward printing.
To prevent vertical recording position misregistration in the forward and backward printing using an MPU, such as described in Japanese Patent Application Publication No. 1-45424, conventionally, fine adjustment is made on output timing of individual print data by software controlling.
According to the prior art, however, it is difficult to achieve high speed printing by hardware controlling since recording position misalignment is adjusted only by the MPU.
Even in the case where the effective speed of the carriage is sufficiently high, rather fine position control of the carriage by the MPU as previously described takes considerable time, and as a result, noticeable improvement in printing speed is not attained.